Storm Sequences Research Articles

Northern Hemisphere Extratropical Cyclone Clustering in ERA5 Reanalysis and the CESM2 Large Ensemble

Abstract Extratropical cyclones are a dominant feature of the midlatitudes, and often occur as storm sequences. This phenomenon is known as cyclone clustering, which is common over regions like the eastern North Atlantic and western Europe. Here, intense clustered cyclones may lead to large cumulative socioeconomic impacts. There are several different approaches to quantify cyclone clustering, but a detailed evaluation on how clustering may change in a warmer climate is missing. We perform a cyclone clustering analysis for the Northern Hemisphere midlatitudes using the ERA5 reanalysis to characterize clustering during 1980–2020. Moreover, we use large ensemble simulations of the Community Earth System Model version 2 following the SSP3-7.0 scenario to compare clustering during 2060–2100 to 1980–2020. Our model simulations show significant enhancement in cyclone clustering over Europe for 3 and 4 cyclones within 7 days in the future decades, which is increasing by up to 25% on average during 2060–2100 compared to 1980–2020. In contrast, cyclone clustering decreases along the west coast of the United States and Canada by up to 24.3% and by 10.1% in the Gulf of Alaska for the same periods. In a warmer climate, clustered cyclones have lower minimum pressure and larger radii and depths compared to nonclustered events. Our findings suggest that change in future cyclone clustering depends on regions affected by global warming, with implications for the cumulative windstorm risk. Significance Statement Storm sequences like the one of December 1999 (Anatol, Lothar, and Martin) have led to large socioeconomic impacts in Europe. It is still unclear how such events will change under global warming. We analyze storm sequences in a reanalysis and a large climate model ensemble for recent (1980–2020) and future climate conditions (2060–2100). Our results show a significant enhancement of storm sequences over Europe for 3 and 4 storms within 7 days, while a decrease is found along the west coast of the United States, western Canada, and in the Gulf of Alaska in future decades. Our findings suggest that the characteristics of cyclone clustering may change in a warmer world, and thus also the associated impacts.

Separating impacts of storm characteristics and mitigation activities on suspended sediment: Implications for sampling to evaluate mitigation success

Mitigation activities are often implemented to reduce anthropogenic sediment loads to sensitive ecosystems, including coral reefs, but effectiveness is rarely documented due to a lack of pre-mitigation data and to the difficulty of separating the impacts of mitigation from those of storm size and intra- and inter-storm variability. Here we present a method using stormwise metrics to quantify the effectiveness of a sediment mitigation project in American Samoa, where an aggregate quarry covering <1 % of a small watershed (drainage area 1.8 km2) generated approximately half of the annual suspended sediment load to the coast. Rainfall, runoff, turbidity and suspended sediment concentrations (SSC, mg L–1), yield (SSY, tons km−2 per unit time) and loads (SSL, tons per unit time) were monitored periodically over a five-year period upstream and downstream of the quarry. Mitigation included revegetation and placement of gravel on roads and processing platforms followed by installation of retention ponds. Storms were identified using automated storm separation, and a simple regression model using maximum area-normalized storm discharge (Qmax) estimated SSC, SSY and SSL for each mitigation stage (post-gravel, post-ponds) for a reference storm sequence. Regression parameters, and SSL and SSC for the reference storm sequence were calculated for a range of storm separation parameters. Monte Carlo sampling of the regression errors was used to calculate a mean and standard deviation of SSL and its change following mitigation. SSC was nearly 100x higher during large storms than during small storms, demonstrating the need to account for storm size to detect impacts of mitigation. After mitigation, SSL downstream of the quarry decreased by 63–82 % for all storms and by 88–92 % for small storms, and SSY downstream of the quarry was similar to SSY from the forested watershed upstream. Mitigation of sediment loads can target a small fraction of a watershed area that accounts for the majority of the human impact. Combining stormwise metrics with simple regression modeling and uncertainty analysis is a simple and novel way to detect and quantify the effectiveness of mitigation activities on sediment loads.

Simulated Atmospheric Response to Large‐Scale Dust Forcing and Implications for Martian Dust Storm Growth

AbstractThe response of tidal, “weather,” and intra‐seasonal transient eddies in a global numerical model to dust imposed in different latitudinal bands and seasons has been examined in order to investigate the impact of regional scale dust storm episodes on large‐scale circulation and hence on further dust storm development. The eddy kinetic energy and surface friction speed for each eddy group were derived using wavelet analysis from multi‐year simulations and statistical comparisons were made among the experiments. Results show that different eddy categories respond differently to dust storm forcing, and that the responses are dependent upon both modeled storm location and season. These responses can be cast in terms of potential positive and negative feedbacks on large‐scale dust storm development and have implications for the cascade of dust storms through different scales and circulation components. The model results suggest positive feedback between northern high latitude dust forcing and weather transients in the same latitudes in Quartober (Ls = 185°–245°), which weakens or disappears in Sixtober (Ls = 295°–360°). The results also suggest positive feedback between dust heating in the tropics/subtropics and tidal eddies, which may enhance the southward transport of “flushing” storms. However, southern high latitude dust forcing suppresses northern weather transients in both pseudo‐season sextons, suggesting negative feedback which may terminate northern frontal/flushing dust storm sequences and hence weaken further development of a dust storm episode through this mechanism.

PROGRESSION OF MEAN DAMAGE ON A MOUND BREAKWATER IN ITS SERVICE LIFE

Because of the expected sea level rise, most of the human properties located on the coast have to be protected during the present century. One of the engineering alternatives is to build rock sloping coastal structures. Currently, their design has to include procedures for aging and deteriorating to forecast their maintenance and reparation costs during its service life. The objective of the present research is to reformulate the rate of mean damage proposed by Melby and Kobayashi (1998) to predict the progression of mean damage, S, on a given mound breakwater typology under sequences of storms with varying wave conditions and water levels. The proposal is based on the interplay of water depth, h, incident significant wave height, Hs and mean wave period, Tm at the toe of the slope, and the impinging number of waves, Nw (Losada et al. (2021)).

COASTAL DUNES CHANGES ALONG THE WESTERN COAST OF EUROPE

Coastal dunes are natural barriers buffering storm waves, protecting coastal communities from flooding and rising sea level, and providing a valuable source of biodiversity for the surrounding environment. Significant dune erosion caused by storm waves and high water levels generally takes place over hours or days, while post-storm recovery can take years or decades (Houser et al., 2015). Although coastal dunes have received quite a lot of attention over the last decades, knowledge gaps remain, and our understanding and predicting capacity of long-term (years to decades) coastal dune evolution remain limited. The large diversity of coastal dunes along the Atlantic coast of Europe and the sequence of extreme storms observed during the 2013/14 winter, considered as the most energetic storms since at least 1948 (Masselink, 2016), represent a unique opportunity to study the spectrum of coastal dune response and recovery from an extreme winter.

BERM MIGRATION UNDER SCALED STORM EVENTS

Berms are one of the natural barriers protecting beaches from wave action and are the first intermittently exposed coastal feature to experience hydrodynamic forcing. Berms erode when storm activity mobilizes sediment offshore or carries it onshore through overwash. However, berm response under varied storm conditions requires further study. Pore water pressure response has been shown to potentially impact beach erosion (Stark, 2022). The infiltration from wave runup followed by exfiltration from rundown destabilize the sediment bed resulting in particle motion. Wave runup increases bed shear stress (Sumer, 2011), and horizontal pressure gradients have been shown to mobilize sediment in the surf zone (e.g. Anderson, 2017). Erosion occurs from numerous physical processes acting together to mobilize the sediment. This study aims to identify these physical processes and berm response for variety of scaled storm sequences.

Storm response and multi-annual recovery of eight coastal dunes spread along the Atlantic coast of Europe

Coastal dunes are natural barriers against coastal flooding, and represent large sources of sediment to mitigate coastal erosion, besides being a natural habitat for many living beings. Yet, these complex environments are threatened by sea level rise and possibly enhanced storminess in the future. Most of the studies on coastal dune erosion and recovery from storms are either site specific or focus on a short-time scale, from months to a couple of years. Here, airborne LiDAR data collected from 2011 to 2020 at eight diverse coastal dunes, spread from NW England to SW France, were analysed to study their response, and recovery from the most energetic extreme storms wave conditions since at least 1948. Results show that the 2013/14 winter was the first or second largest erosive event (from −14 to −290 m3/m dune volume loss) from 2011 to 2020 at all sites. The magnitude of storm-driven sand volume loss was mainly controlled by dune face slope (r = 0.84). Dunes with steeper pre-storm slopes lost the largest volumes of sand. At a dune scale, the scarping height was also well correlated to the dune face slope at sites where storm response was characterized by limited alongshore variability. Dune recovery was site specific (no recovery, partial, complete, excess), with dunes that either progressively returned to their pre-storm morphology or were reshaped while recovering. Percentage of dune sand volume recovery was well correlated to the local and long-term satellite-derived shoreline change rate computed from 1984 to 2021 (r = 0.81), suggesting that dune recovery is mainly controlled by the local coastal sediment budget. The rate of dune crest elevation increase (from 4.2 to 12 cm/year) at four of the study sites from 2011 to 2020, largely exceeded sea level rise rate over the past decade (3.3 ± 0.7 mm/year). These results provide key insight into the contrasting resilience of some of the most exposed coastal dunes along the Atlantic coast that recover at different rates following the same sequence of extreme storms.

Inter-annual morphological evolution and volume changes of a meso- to macrotidal beach exhibiting multiple intertidal bars (MITB)

Although morphologically persistent in the long term, Multiple Intertidal Bar Systems (MITBs) display short-term, especially seasonal, morphodynamic behaviour. Analysis of high-density, monthly DGPS surveys conducted at Murlough and Ballykinler beaches, inter- and supratidal sediment volumes and hydrodynamic forcing (wave conditions and water levels), demonstrates a link between strong seasonality in wave climate and MITB beach behaviour. Summer, low-energy conditions limit cross-shore sediment exchange during which MITB beach morphology tends to stabilise throughout the season. With the onset of high-energy winter conditions cross-shore sediment exchanges occur between inter- and supratidal areas. Sediment transport is then enhanced during storm conditions that are coincident with spring tides, leading to high total water levels (TWL). Ultimately the storm sequencing, (frequency, magnitude and inter-storm interval), is the key parameter driving the beach morphological response. Major erosional patterns occur when the most energetic event, combined with spring high tide, occurs at the beginning of the winter season. Subsequent, less energetic storms then promote bar recovery until another extreme event occurs.Seasonality is also evident in alongshore dynamics. Cross-shore drainage channels that dissect the intertidal bars migrate alongshore, driving alongshore sediment transport and MITB longshore migration patterns. In summer migration of drainage channels is limited, whereas the winter high-energy forcing enhances channel migration rates and resulting sediment transport. Differences in dynamics between the two study sites are attributed to differences in local geology, beach morphology and sediment size, but in both locations, drainage channels are in fine migrating toward the inlet and associated ebb delta that divides the bay. Subsequent transport vectors are unknown, but the observations highlight the primary role of the inlet in the sediment circulation dynamics in the system as a whole.

Martian dust storm distribution and annual cycle from Mars daily global map observations

Dust storms were manually tracked in Mars Reconnaissance Orbiter (MRO) Mars Daily Global Maps (MDGMs) from Mars Year 29 to 33. The data were used to construct the Mars Dust Storm Sequence Dataset (MDSSD), which contains >12,000 dust storm instances that are distinguishable from the ubiquitous dust background. Based on the dust storm climatology, we propose a partition of a Mars year into six pseudo-seasons or “sextons” (named “Unober”, “Duober”, “Triober”, “Quartober”, “Quintober”, and “Sixtober”, respectively). The greatest dust storm activity is observed in the 3rd, 4th and 6th sextons (Triober, Quartober and Sixtober), with each containing one or more dust storm episodes. We also propose a hierarchical system for organizing information and describing the process through which dust storms develop. The hierarchy of storm behavior is described in terms of dust storm episodes within these sextons, with each episode being composed of one or more dust storm sequences, and with each dust storm sequence being composed of multiple dust storm members.The occurrence of a large dust storm episode typically results from multiple dust storm sequences that are simultaneous, staggered, or merged, though one of the component sequences typically plays a dominant role. The typical evolutionary pathway through which a large dust storm sequence develops consists of dust activity progressing from the northern to the southern hemispheres, with multiple flushing dust storm members in the northern hemisphere and followed by a much larger zonally extended dust storm member in the southern hemisphere. Differences among Mars years are related to the timing, order, and trajectories of dust storm sequences, though the overall spatial coverage of dust storm instances within a sexton is similar across years. A striking characteristic of Martian dust storm distribution is the quasi-periodic recurrence exhibited by dust storm episodes and dust storm sequences.

Storm sequence chronology and initial profile morphology controls on beach erosion

Projected changes in frequency and intensity of extreme storms have been widely reported in the literature. While individual storms can transport large amounts of sand creating significant beach erosion, affecting communities, and damaging infrastructure, storm groups that happen close together without time for the beach to recover can further intensify these impacts. The aim of this study is to quantify the total beach erosion induced by a storm group and the importance of the storm sequence chronology (SC) on the total eroded volume. To achieve this, a morphological numerical model was applied to analyze the response of three different initial beach profiles. A measured storm group of four storms was simulated and the storm order were rearranged to assess its importance to total beach erosion. The results showed that the beach continued to erode unless the wave power of current storm was at least 76% (on average) lower than the previous storm, and that the relationship between total water level and erosion rate were inversely proportional while the beach volume was over 80% of its pre-storm volume. This relationship became directly proportional when the volume was below 80% of its pre-storm volume. This study assesses the beach erosion depending on combination of different storm order and pre-storm beach volume at beaches with varying wave attenuation.

Natural and nature based features for environmental enhancement and coastal storm risk management: a case study on Marco Island, Florida, United States

Natural and Nature-Based systems provide an opportunity for adaptive response to Coastal Storm Risk Management and Sea Level Rise. The Tigertail Lagoon/Sand Dollar Island Restoration on Marco Island, Florida, presents a case study designed to maintain and enhance an existing coastal barrier system consisting of a 3-km-long sand spit and tidal lagoon ecosystem that is otherwise evolving toward closure. The case study is part of a nature-based adaptive management plan to restore and stabilize the sandspit and tidal lagoon through cyclic use of sediment within the system. This approach seeks to preserve existing protective habitats and landforms that also serve as natural coastal barriers to protect upland development. Design of the restoration plan considers the functions of a wildlife nature preserve and evolution of complex tidal inlet morphologic features bordered by a heavily developed barrier Island. The design aims to restore and enhance a sandspit degraded by a sequence of storms since Major Hurricane Irma impacted Southwest Florida in 2017 and improve the existing deteriorated habitat by enhancing tidal exchange through restoration of the lagoon flow channel. Total wetland area will be increased by relocating the sand spit seaward of its present location to where it was located in approximately 2017. The reconstructed beach berm will provide enhanced resiliency to high frequency weather events. Sediment will be sourced from the existing sand spit and an innovative sand trap that will maintain the lagoon entrance open while providing beneficial re-use for excess sediment that continues to accumulate at the end of the spit. Components of the project were analyzed using existing engineering models and methods such as the Coastal Modeling System (CMS) and XBeach. Enhancing and preserving this barrier island feature and productive ecosystem provides an example of the enhanced coastal resiliency provided by natural and nature-based systems that are adaptable and responsive to sea level rise and ongoing coastal processes.

Wave overtopping at near-vertical seawalls: Influence of foreshore evolution during storms

This work presents the results of an investigation on how wave overtopping at a near-vertical seawall at the back of a sandy foreshore is influenced by sequences of erosive storms. The experiments were carried out in the Large Wave Flume (GWK) at Leibniz University, Hannover (Germany). The tested layout consisted of a near-vertical 10/1 seawall and a sandy foreshore with an initial 1/15 slope. Three sequences of idealised erosive storms were simulated. Within each storm both the incident wave conditions and still water level were varied in time to represent high and low tide conditions. Each sequence started from a 1/15 configuration and the beach was not restored in between storms. The measurements included waves, beach profile, wave overtopping volumes. The profile of the beach was measured after each sea state tested.Wave overtopping at each stage of the tested storms was significantly influenced by bed changes. This was linked to the measured evolution of the beach. Measurements showed that a barred profile developed quickly at the start of each sequence, and scour developed at the toe of the structure during high water level conditions, while accretion or partial backfilling developed during low water level conditions. Due to these processes, the position of a sea state in the tested sequence is shown to be an important factor in determining the wave overtopping volume. Remarkably, when a weaker idealised storm followed a more energetic one, nearly the same level of overtopping was recorded. This is explained by the foreshore erosion, leading to increased water depths and wave heights at the toe of the structure. This finding allows to quantify and to explain the variability of wave overtopping in storms following one another at intervals shorter than the recovery time of the foreshore.

Climate change is increasing the risk of a California megaflood

Despite the recent prevalence of severe drought, California faces a broadly underappreciated risk of severe floods. Here, we investigate the physical characteristics of “plausible worst case scenario” extreme storm sequences capable of giving rise to “megaflood” conditions using a combination of climate model data and high-resolution weather modeling. Using the data from the Community Earth System Model Large Ensemble, we find that climate change has already doubled the likelihood of an event capable of producing catastrophic flooding, but larger future increases are likely due to continued warming. We further find that runoff in the future extreme storm scenario is 200 to 400% greater than historical values in the Sierra Nevada because of increased precipitation rates and decreased snow fraction. These findings have direct implications for flood and emergency management, as well as broader implications for hazard mitigation and climate adaptation activities.

Emulator For Eroded Beach And Dune Profiles Due To Storms

AbstractDunes and beaches are vulnerable to erosion during storm events. Numerical models can predict beach response to storms with fidelity, but their computational costs, the domain‐specific knowledge necessary to use them, and the wide range of potential future storm and beach conditions can hinder their use in forecasting storm erosion for short‐ and long‐term horizons. We develop an emulator, which is an efficient predictive model that behaves like a numerical model, to predict the morphologic response of the subaerial beach to storms. Specific emphasis is placed on providing antecedent beach states as an input to the emulator and predicting the post‐storm profile shape. Training data include beach profiles at multiple stages in a nourishment life cycle to assess if such a framework can be applied in locations that nourish as a coastal defense policy. Development and application of the emulator is focused on Nags Head, North Carolina, which nourishes its beaches to mitigate hazards of storm waves, flooding, and erosion. A high‐fidelity, process‐based morphodynamic model is used to train the emulator with 1250 scenarios of sea‐storms and beach profiles. The post‐storm beach state is emulated with a parameterized power‐law function fit to the eroded portion of the subaerial profile. When the emulator was tested for a sequence of real storms from 2019, the eroded beach profiles were predicted with a skill score of 0.66. This emulator is promising for future efforts to predict storm‐induced beach erosion in hazard warnings or adaptation studies.

Single extreme storm sequence can offset decades of shoreline retreat projected to result from sea-level rise

Extreme storms cause extensive beach-dune erosion and are typically considered to enhance coastal erosion due to sea-level rise. However, extreme storms can also have a positive contribution to the nearshore sediment budget by exchanging sediment between the lower and upper shoreface and/or between adjacent headlands, potentially mitigating some adverse sea-level rise impacts. Here we use three high-resolution morphological datasets of extreme storm-recovery sequences from Australia, the UK and Mexico to quantify the nearshore sediment budget and relate these episodic volume changes to long-term coastal projections. We show that sediment gains over the upper shoreface were large (59–140 m3/m) and sufficient to theoretically offset decades of projected shoreline retreat due to sea-level rise, even for a high-end greenhouse gas emissions scenario (SSP5-8.5). We conclude that increased confidence in shoreline projections relies fundamentally on a robust quantitative understanding of the sediment budget, including any major short-term sediment contribution by extreme storms.

Spatio-Temporal Analysis of Dust Storm Activity in Chryse Planitia Using MGS-MOC Observations from Mars Years 24–28

Dust storms, observed in all seasons, are among the most momentous of Mars’ atmospheric activities. The Entry–Descent–Landing (EDL) activity of a Martian landing mission is influenced by local atmospheric conditions, especially the probability of dust storm activity. Chryse Planitia, featuring many of the largest and most prominent outflow channels and possible mud volcanoes, is an important target site for current and future Mars landing missions. It is of great significance to understand that a Mars landing probe may encounter a dust storm situation during EDL season in the Chryse Planitia. In this study, based on four Martian years, Mars Orbiter Camera (MOC) Mars Daily Global Maps (MDGMs), 1172 dust storms were identified within Chryse’s 1600 km-radius ring. Secondly, the daily mean dust storm probability was calculated, binned by 1° of solar longitude in the Chryse landing area. The two active periods of dust storm activity are Ls = 177–239° and Ls = 288–4°, with an average daily mean dust storm probability of 9.5% and 4.1%. Dust storm activity frequency is closely interrelated with the seasonal ebb and flow of the north polar ice cap; consequently, most dust storms occur in either the cap’s growth or recession phase. We divided the Chryse landing area into square grids of 0.5° and computed the average probability of dust storm occurrence in each grid, which ranged from 0.19% to 2.42%, with an average of 1.22%. The dust storm activity probability in space was also inhomogeneous—low in the west and south but high in the east and north—which was mainly affected by the origin and the path of dust storm sequences. Based on empirical orthogonal function (EOF) analysis of storms in the Chryse area, 40.5% are cap-edge storms in the northern hemisphere. Finally, we concluded that the preferred time of a Mars landing mission is Ls = 18–65° in the Chryse Planitia, and three preferred landing areas were selected with low dust storm probability.

Emergent coastal behaviour results in extreme dune erosion decoupled from hydrodynamic forcing

Coastal dune systems provide vital natural barriers against storm impacts and coastal inundation. In times of rising sea levels and uncertainty over increasing storminess, it is critical that dune erosion is adequately understood and actively monitored. This study investigates the severe erosion of the climbing dune system at Crantock, an exposed macro-tidal beach in north Cornwall, UK, that before 2013 showed relative stability. In contrast to regional consistency in beach recovery across north Cornwall since the major storms of 2013/14, Crantock beach and dune system have shown an acceleration in erosion. This has resulted in dramatic cut-back of the front of the climbing dune system since 2016, despite the reduced frequency of severe storm events since 2013/14. The decoupled nature and emergent response of Crantock's dune system are explained by the shifting channel of the River Gannel, which has its outflow over the beach. Intertidal bar movement during the recovery from the 2013/14 storm sequence, alongside an ongoing deterioration of the training wall that pinned the River Gannel to the East Pentire cliffs to the north of the beach, has led to a southward avulsion of the river that has since lowered the elevation of the beach in front of the dunes. XBeach modelling suggests that the increased dune erosion can be attributed to a lowering of the beach profile and steepening of the dune face, indicating that the river avulsion has triggered a step-change in the dune equilibrium and the onset of dramatic erosional events.

The extreme space weather events in October 1788

Abstract Solar eruptions launch interplanetary coronal mass ejections (ICMEs) and cause geomagnetic storms and equatorial extension of the auroral oval. Their rare and unique nature has made analyses of historical events extremely important to increase their data availability. In this study, we analyzed the space weather event of 1788 October, which was characterized with simultaneous auroral observations. We extended archival surveys and confirmed the auroral visibilities down to Barcelona (46.0° MLAT) on October 21/22 as well as Mizuhara (27.5° MLAT) and Rome (44.8° MLAT) on October 22/23. The end of auroral reports overlapped with a reported declination disturbance at Mannheim, indicating a ΔD amplitude of ≥1.15°. Two positive excursions of ΔD were recorded, lasting for several tens of minutes. Upward field-aligned currents could have flowed poleward of Mannheim associated with substorms. We identified the equatorial boundary of the auroral oval down to 46.5° ILAT in the European sector and approximately ≤41.6° ILAT in the Japanese sector. This is compared with the reported equatorial auroral boundaries during extreme storms. The long storm sequence indicates the arrival of multiple ICMEs, thereby enhancing solar activity at that time. This sequence is indeed contextualized immediately after the maximum of Solar Cycle 4. Because sunspot observations are extremely scarce around 1788, it is challenging to identify the source active region. This in turn makes these auroral records valuable for the analyses of long-term solar activity before the onset of the Dalton Minimum.

The extreme 2013/14 winter storms: Regional patterns in multi-annual beach recovery

Understanding the mechanisms and timescales required for beaches to recover from extreme storm events is fundamental for coastal management worldwide. Yet the post-storm recovery characteristics of different beach types have rarely been investigated over multi-annual timescales. Previous work along the southwest coast of England has suggested that the magnitude and alongshore variability of beach response to storms can be grouped into four key response types controlled by the level of exposure, angle of storm wave approach and degree of embaymentisation. This study aims to enhance our understanding of the spatial and temporal patterns of post-storm beach recovery within this storm response classification framework. Analysis was based on morphological survey data from 23 sites along the southwest coast of England collected between 2012 and 2017. We found that beaches that responded similarly to the unprecedented storm sequence of 2013/14, recovered in a similar manner too, and that spatio-temporal patterns of post-storm recovery can, for the most part, also be described by four coherent classes. In terms of complete recovery to pre-2013/14 volumes, 7 of the 23 beaches we studied recovered >90% of their sediment within 3 years or less, including some of the most affected sites. The magnitude of intertidal beach volume recovered (in the order of 1-100m3m-1) was well correlated with the storm erosion volume (R = -0.81, p = 0.00) and, importantly, was similar for beaches within the same response class. Fully exposed, cross-shore dominated beaches experienced the highest gross erosion and recovery volumes, but showed the lowest net recovery after 3 years (median: 74% volume recovered), while semi-exposed cross-shore dominated beaches showed lower gross change, but the highest net recovery (median: 93% volume recovered).In most cases, the spatial pattern of recovery mirrored that of the storm impact, regardless of whether the beach was cross-shore or alongshore dominant. The observed coherency within each of the four studied beach response classes indicates that regional monitoring programmes could make considerable cost savings by strategically targeting monitoring at representative sites within each class, rather than monitoring all beaches within a region.

Nitrogen/phosphorus behavior traits and implications during storm events in a semi-arid mountainous watershed

Seasonal rainfall events reinforce the link between terrestrial and fluvial domains and are crucial for assessing hydrological control over riverine nutrient dynamics and pollutant source behaviors, especially in a semi-arid watershed. Taking the Qingshuihe river basin, a semi-arid mountainous basin in China, as an example, this paper investigated storm effects on riverine nitrogen (N) and phosphorus (P) dynamics (i.e. concentration, load, and composition changes) through continuous sampling of four storm events of the 2019 rainy season, including one small storm, two moderate storms, and a large storm. Pollutant sources and transport pathways were then examined over the storm sequence via hysteresis analysis. The results revealed a strong linkage between N/P dynamics and hydrological processes. Storm runoff caused a 6-fold increase in particulate-P (PP) and a 4-fold increase in ammonia-N (NH4-N) fluxes through four storms (most sensitive nutrients to storms). On average, PP shared 86% of P exports, and nitrate-N (NO3-N) contributed 79% of N exports. PP and NH4-N were delivered primarily from overland sources and transported by surface runoff. Nonetheless, mobilization of channel sediment reserves was also an important way of PP supply during storms. The results suggested groundwater as the principal NO3-N source in the watershed, and subsurface flow was important for NO3-N and total dissolved-P (TDP) delivery during storms. The large storm (>20 mm) often registered the highest N/P load exports. However, there were other influencing factors/processes on stormflow N/P dynamics in the semi-arid watershed, which complicate/override the effects of different storm magnitudes. Total suspended solids (TSS)/PP source availability and inter- and intra-storm export trends influenced P behaviors through storms. Moreover, impacts of mobilization processes on NO3-N behavior appeared over the storm sequence. These findings enhance our understanding of storm events induced N/P exports in water-scarce regions and provide references for water quality predictions and control in flood seasons.